The present invention relates to a transfer-type thermal printer for thermally transferring a thermally fusible ink on to a recording paper, and a thermal transfer printing method.
A thermal fusion transfer recording carried out by a transfer-type thermal printer is a kind of non-impact recording system for transferring ink which is solid in an ordinary state on to a recording paper for forming a visible pattern by using Joule heat as a recording energy.
According to the thermal fusion transfer recording technique, at first, an ink ribbon having a solid ink layer and a recording paper are superposed with each other, and the ink ribbon and the recording paper are sandwiched between a thermal head and a platen. Next, electric power is applied according to an image signal to a fine heating unit provided on the thermal head, to heat the heating unit. With this heat, the solid ink layer of the ink ribbon is fused and is then adhered on to the recording paper. When the ink ribbon is removed from the recording paper, an ink layer having a desired pattern is transferred on to the recording paper.
The ink ribbon generally consists of an ink layer (1 to 6 .mu.m, that is about 1 to 6g/m.sup.2), a base film (2 to 6 .mu.m) and a heat-resistant (lubricating) layer (1 to 3 .mu.m). The ink layer is structured by a coloring agent selected from pigments and dyes, for example, a binder selected from waxes and thermoplastic resins, for example, and various types of additives selected from softening agents and dispersants, for example.
As the base film, polyethyleneterephthalate (hereinafter to be referred to as PET) is mainly used. This PET is softened and fused at 263.degree. C. On the other hand, the surface temperature of the heating unit of the thermal head exceeds 300.degree. C. The heat-resistant layer is provided between the thermal head and the PET in order to prevent an occurrence of a phenomenon called a stick phenomenon in which the ink ribbon is fused to the thermal head to make it difficult to move the ink ribbon. Further, the heat-resistant layer can also have the effect of lubricating properties and antistatic properties.
The binder which occupies 60 to 80 wt % of the ink component determines the thermal fusion characteristics of the ink. In general, the ink of a wax-based binder has such characteristics as a sharp range of melting point and a rapid reduction in viscosity at this melting point or above. On the other hand, the ink of a thermally-fusible resin based binder shows a broad range of melting points and a smooth reduction in viscosity at this melting point or above. The ink having a broad range of melting points can be transferred uniformly even if there are some temperature variations within a heating area. On the other hand, the ink having a sharp range of melting point is excellent in sharpness of an image edge.
A schematic diagram of a part of an example of the thermal head used for the transfer-type thermal printer is shown in FIG. 1. FIG. 2 is a diagram showing a partially cut enlarged portion 10 of FIG. 1. Generally, the thermal head is provided by forming a heat resistor array 6 structured by Ta.sub.2 N, RuO.sub.2, BaRuO.sub.3, etc., for example, on a substrate 5 structured by ceramics or the like, by using a thin film process of evaporation or a thick film process of screen printing or the like, for example. In order to improve the heat insulating and an adhesive properties, a glazed layer 7 can also be formed between the array resistor 6 and the substrate 5. Further, the whole of the thermal head is covered by wear resistant layer 8 structured by TaO.sub.5, SiN and SiC, for example. A heat discharging panel 9 is also provided on the lower surface of the substrate 5.
For carrying out a color printing by using the thermal head as described above, an ink layer of a first color and a recording paper are sandwiched between the thermal head and the platen, and the ink layer of the first color is transfer printed. Then, the recording paper onto which the ink layer of the first color has been transferred and an ink layer of a second color are superposed and sandwiched between the thermal head and the platen, for printing the ink of the second color on the surface of the recording paper on which the ink of the first color has been printed. Usually, three original colors of Y (yellow), M (magenta) and C (cyan) are used for the color printing, so that transfer printing is repeated at least three times. In this case, in order to obtain desired colors, three kinds of inks are printed to be superposed by suitably repeating the printing.
For the above-described printing, a plane smooth paper called a thermal paper is used exclusively. Usually, a coated layer is provided on this thermal paper for an improved whiteness and for an improved fixing properties of the inks. However, even if this kind of thermal paper is used, it is difficult to superimpose an ink at completely the same position on the paper, with a result that the ink is usually printed with a deviation of about several ten um.
FIG. 3A, FIG. 3B and FIG. 3C are schematic diagrams for showing a status wherein the three colors of Y. M and C are sequentially transferred in this order in one dot on a smooth recording paper 1. The upper diagram connected with dotted lines is a schematic view from the top direction and the lower diagram is a schematic view from the transverse direction. FIG. 3A shows the case where the colors are transferred accurately to the same position. FIG. 3B and FIG. 3C show virtual model diagrams or actual model diagrams of the case where the colors are transferred, with M and C deviated from the position of Y by about several ten .mu.m, respectively. When M and C are transferred with a deviation of about several ten .mu.m from the position of Y, the colors are considered to be usually transferred as shown in FIG. 3B. However, since there occurs a difference in level between the first layer and the printing sheet, a part of M and C is chipped respectively so that the colors are transferred as shown in FIG. 3C. When there arises a deviation in the transfer positions of the three colors, as explained above, the transfer of M and C to be superposed on Y becomes insufficient.
FIGS. 4A and FIG. 4B are schematic diagrams for showing a status wherein the three colors of Y, M and C are transferred in this order in three dots on the smooth recording paper 1. FIG. 4A shows a virtual model diagram of the case where the three dots are transferred to accurate positions. FIG. 4B shows an actual model diagram of the case where the dot of C is printed between the two dots which are the superimposition of M on Y. Essentially, when one dot is formed between the two dots formed with a distance of one dot space therebetween, the colors are considered to be transferred as shown in FIG. 4A. However, in actual practice, because of a difference in level formed between the transferred ink layers of two dots and the recording paper, the transfer of the third color becomes insufficient as shown in FIG. 4B.
The above explanation relates to cases wherein inks of three colors are used, but a transfer failure as described above occurs frequently even if inks of two colors are transferred when the ink transfer quantity is not less than 2 g/m.sup.2. Further, in the case of superpositioning of three colors, this kind of transfer failure occurs even if the ink transfer quantity is 1 g/m.sup.2.
As explained above, conventionally when a color printing is carried out using a smooth recording paper, there arises a difference in level between the ink layer of the first color and the surface of the recording paper, so that the transfer of the ink of the second color can become unstable. Further, in the worst case, a dot of the ink of the second color is formed on only the ink of the first color.
In the case of expressing a halftone by varying a dot size, there is a possibility that case where the dot of the second color will be larger than the dot of the first color even if no positional deviation occurs. In this case, there also arises a problem in that the transfer of the ink of the second color becomes unstable, with a result that the color transfer is not carried out satisfactorily.
Further, when three colors are superposed together or when four colors are superposed together using a black color, the ink transfer becomes more unstable so that the transfer can not be carried out satisfactorily.
In the literature, "Multi-gradation Thermal Transfer Printing according to a Fusible Ink Permeation System", (a technical report of The Institute of Television Engineers of Japan, Vol. 17, No. 27, PP. 19-24, VIR '93-28 May, 1993), there is a description that when a recording paper having a large number of pores on its surface is used, an ink is permeated into the recording paper to ensure a stable transfer.
When a recording paper having many pores on its surface as described in this literature, "Multi-tone Thermal Transfer Printing according to a Fusible Ink Permeation System", was used to carry out a high-speed color printing with a resolution 300 dpi, it was made clear that a transfer failure occurs when a first color (Y), a second color (M) and a third color (C) are transferred sequentially in superposition.
Microphotographs for expressing this status are shown in FIG. 5, FIG. 6 and FIG. 7. FIG. 5 is a microphotograph showing the surface of the recording paper printed with the first color (Y), which is enlarged to 300 times magnification, and FIG. 6 is a microphotograph showing the surface of the recording paper which is further enlarged to 1000 times magnification. From both FIG. 5 and FIG. 6, it is clear that the ink of the first color (Y) is not permeated sufficiently into the surface of the recording paper. FIG. 7 is a microphotograph showing the surface of the recording paper printed with the three colors in superposition, which is enlarged to 100 times. From FIG. 7, it is known that since the permeation of the ink of the first color is insufficient, the dots of the second color (M) and the third color (C) are chipped due to the difference in level between the first color (Y) layer and the recording paper so that a transfer failure occurred. This phenomenon similarly occurs when the resolution is as high as 600 dpi.
As explained above, with a mere provision of pores on the surface of the recording paper, an ink transfer failure may occur in the second color, thereafter resulting in an insufficient color transfer.
Further, there is also a problem in that, in order to achieve a sufficient permeation of inks, it is necessary to fuse the inks sufficiently, which leads to an increasing of the energy to be applied to the head.
Further, according to the conventional transfer-type thermal printer, there is a problem that it is difficult to increase the speed of printing, particularly in the case of a color printing, so that it is difficult to apply the transfer-type thermal printer to a multi-gradation color printing.
The inventors of the present invention studied the causes of the ink transfer failure which occurred in the conventional thermal transfer printing using a recording paper provided with a large number of pores on the surface of the recording paper as described in literature, "Multi-gradation Thermal Transfer Printing according to a Fusible Ink Permeation System". As a result of this study, it was made clear that the causes of the ink transfer failure are a poor balance between an ink quantity to be transferred and an acceptable quantity of permeated ink, a slow permeation speed due to low temperature and high viscosity of inks, a short permeation time due to high speed of printing, etc.
To solve the above problems, various measures can be considered such as a delaying of the printing speed and lowering of the ink viscosity by increasing the applied energy, for example. However, the delaying of the printing speed has a problem of making it impossible to achieve the high-speed printing. Further, the increasing of the applied energy also has a problem of making it impossible to achieve a multi-gradation color printing.